Many of the conventional photoinitiators or photoinitiator systems limit the applicability of photopolymerizable compositions because they are activatable primarily by radiation in the ultraviolet or shorter wavelength region (i.e., blue and green) of the visible spectrum. The availability of reliable, relatively inexpensive lasers which emit in the longer wavelength (i.e., red) region of the visible spectrum and which can be used as output devices for electronic imaging systems has made it desirable to develop photoinitiator systems which are activatable by this spectral region. Applications for photopolymerizable compositions containing these photoinitiator systems include graphic arts films, proofing, printing plates, and photoresists. The preparation of holograms in photopolymerizable compositions also requires photoinitiator systems activatable by radiation in this spectral region.
Photopolymerizable compositions containing 2,2',4,4',5,5'-hexaarylbisimidazoies, or HABI's, are well known. Sensitizers which extend the sensitivity of these compositions are also known. Baum, U.S. Pat. No. 3,652,275, discloses photopolymerizable compositions containing selected bis(p-dialkylaminobenzylidene)-ketones as sensitizers of HABI photoinitiator systems. Dueber, U.S. Pat. No. 4,162,162, discloses selected sensitizers derived from aryl ketones and p-dialkylaminoaldehydes. Smothers, U.S. Pat. No. 4,917,977, and Monroe, U.S. Pat. No. 4,987,230, also disclose visible sensitizers for photopolymerizable compositions. However, despite the advances which have been made, there is a continuing need for photoinitiator systems which are sensitive to the visible region of the spectrum.